1. Field of the Invention
The invention relates generally to a method and apparatus for extruding single or multiple layers of plastic and specifically to a method and apparatus for extruding single or multiple layers of plastic on an electrical conductor or for forming other plastic articles without having to adjust the concentricity of layers every time the tooling is changed to make different size coatings or to coat different size wires.
2. Prior Art
Manufacturing electrical wires with single or multiple layers of plastics is a well known art. The problem encountered so far in achieving uniform coatings of plastic on the wire is in getting the plastic layers around the conductor to be concentric. So far there have been two different approaches taken to obtain concentric layers. The first approach is commonly known in the industry as an Adjustable Head. Using an Adjustable Head, every time a different product is made, one has to adjust the positions of the tip or die in relation to each other to achieve a concentric layer of plastic. This adjustment requires a skilled operator, takes up valuable production time, and while the adjustments are being made both plastic and wire is being wasted.
The second approach is commonly known in the industry as Fixed Center. In Fixed Center, one eliminates the need for adjustments by making all of the parts which guide the wire through the middle of the extruder and the plastic around it concentric and hoping this will result in concentric layers of plastic. This approach saves adjustment time and no skilled operator is needed, except one can not be sure of getting concentric layers of plastic due to the fact that, even with present technology, one can not guarantee that all the parts making up the device are suitably concentric, as there must be some tolerance in the sizes of parts which slide into each other, and the flow characteristics of different plastics is different. All of these variables result in some eccentricity of the layers of plastics and there is no means provided to make the minor adjustments.
The patent to Weber, U.S. Pat. No. 4,838,777, discloses a more or less typical adjustable head extrusion means for coating wire. In the Weber invention, the entire die base ring is adjusted relative to the extrusion support, with the entire die assembly being adjusted relative to the tip of the wire guide. One disadvantage of the Weber device is that the entire die assembly must be adjusted to create a uniform coating on the wire, thus allowing the possibility of moving the die assembly so that it is no longer coaxial with the wire guide and creating the necessity of readjusting the entire assembly as alternate dies are placed within the assembly.
The patent to Starnes, Jr., U.S. Pat. No. 4,773,954, discloses a method and apparatus for simultaneously extruding two coatings onto an electric wire. Generally, the Starnes device is configured to allow the coating of two different types of plastic on the electric wire, generally plastics with two different melting points. The Starnes device lacks an adjustment mechanism for adjusting the application of either coating and, therefore, suffers the disadvantage of possibly applying uneven coatings onto the electric wire.
The patent to Wheeler, Jr., U.S. Pat. No. 4,280,801, discloses a cross-head for an extruding device which is capable of applying multiple layers of coating to a wire. The dies for forming each layer of coating need to be adjusted relative to each other every time any of the tips or dies are changed to change the product size, therefore, having all of the disadvantages of a conventional adjustable head. Another disadvantage of the Wheeler device is the use of multiple dies, or tools, in order to apply the multiple layers of plastic on the wire. The use of multiple dies adds complexity and cost to the extruding machine and, if unadjustable or improperly adjusted, allows for the uneven coating of multiple layers on the wire. Further, if it is desired to replace one of the dies, the entire mechanism must be dismantled and readjusted upon rebuilding.
The patents to Meyers, U.S. Pat. No. 3,947,172, Dougherty, U.S. Pat. No. 3,903,233, and Toomajanian, U.S. Pat. No. 3,820,927, all disclose apparatus for applying one or more coatings to a moving wire. Each of these patents suffer the same disadvantages as those described above, namely, lack of an adjustment mechanism separate from the die and the use of multiple dies.